U.S. patent application number 14/592351 was filed with the patent office on 2015-05-07 for device and method for communicating channel state information reference signal (csi-rs) in wireless communication system.
The applicant listed for this patent is Samsung Electronics Co., Ltd.. Invention is credited to Shan CHENG, Jin Kyu HAN, Sung Tae KIM, Youn Sun KIM, In Ho LEE, Myung Hoon YEON.
Application Number | 20150124758 14/592351 |
Document ID | / |
Family ID | 44921190 |
Filed Date | 2015-05-07 |
United States Patent
Application |
20150124758 |
Kind Code |
A1 |
KIM; Youn Sun ; et
al. |
May 7, 2015 |
DEVICE AND METHOD FOR COMMUNICATING CHANNEL STATE INFORMATION
REFERENCE SIGNAL (CSI-RS) IN WIRELESS COMMUNICATION SYSTEM
Abstract
A method for wireless communication by a terminal, a method for
wireless communication by a base station, the terminal, and the
base station, are provided. The method for wireless communication
by the terminal includes receiving first information comprising a
muting subframe interval, a subframe offset, and a muting position
of a resource element in a resource block, checking presence of a
data in a subframe, determining the resource element to be muted in
the subframe based on the muting subframe interval, the subframe
offset, and the muting position, if the data is present, and
receiving the data on a physical downlink shared channel (PDSCH)
based on the result of the determining step.
Inventors: |
KIM; Youn Sun; (Seongnam-si,
KR) ; HAN; Jin Kyu; (Seoul, KR) ; KIM; Sung
Tae; (Suwon-si, KR) ; YEON; Myung Hoon;
(Yongin-si, KR) ; CHENG; Shan; (Suwon-si, KR)
; LEE; In Ho; (Suwon-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Samsung Electronics Co., Ltd. |
Suwon-si |
|
KR |
|
|
Family ID: |
44921190 |
Appl. No.: |
14/592351 |
Filed: |
January 8, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14154770 |
Jan 14, 2014 |
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14592351 |
|
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12987436 |
Jan 10, 2011 |
8634363 |
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14154770 |
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Current U.S.
Class: |
370/329 |
Current CPC
Class: |
H04L 1/0027 20130101;
H04L 1/0693 20130101; H04L 5/0053 20130101; H04L 5/0092 20130101;
H04L 5/0057 20130101; H04L 2025/03802 20130101; H04L 5/0035
20130101; H04B 17/24 20150115; H04L 1/0026 20130101; H04L 5/0023
20130101; H04W 72/04 20130101; H04L 2025/03426 20130101; H04L
1/0003 20130101; H04L 1/0009 20130101; H04L 5/0073 20130101; H04L
25/0226 20130101; H04W 72/042 20130101; H04L 1/06 20130101; H04L
5/0082 20130101; H04L 5/0048 20130101; H04W 72/085 20130101; H04W
72/1226 20130101; H04B 17/318 20150115; Y02D 30/70 20200801 |
Class at
Publication: |
370/329 |
International
Class: |
H04W 72/04 20060101
H04W072/04; H04W 72/08 20060101 H04W072/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 12, 2010 |
KR |
10-2010-0002837 |
Feb 11, 2010 |
KR |
10-2010-0013005 |
Claims
1. A method for wireless communication by a terminal, comprising:
receiving first information comprising a muting subframe interval,
a subframe offset, and a muting position of a resource element in a
resource block; checking presence of a data in a subframe;
determining the resource element to be muted in the subframe based
on the muting subframe interval, the subframe offset, and the
muting position, if the data is present; and receiving the data on
a physical downlink shared channel (PDSCH) based on the result of
the determining step.
2. The method of claim 1, wherein the data is received based on the
muting position if the resource element in the subframe is to be
muted.
3. The method of claim 2, wherein the data is received on PDSCH
except for the muting position, and the data is not received on the
muting position.
4. The method of claim 2, wherein the data is mapped on PDSCH
except for the muting position, and the data is not mapped on the
muting position.
5. The method of claim 1, wherein the first information is
generated based on second information for neighbor cell.
6. The method of claim 5, wherein the second information comprises
channel state information reference signal (CSI-RS) pattern for
neighbor cell.
7. A method for wireless communication by a base station,
comprising: generating first information comprising a muting
subframe interval, a subframe offset, and a muting position of a
resource element in a resource block; transmitting the first
information to a terminal; and transmitting a data on a physical
downlink shared channel (PDSCH) based on the resource element to be
muted in a subframe, wherein the resource element is determined
based on the muting subframe interval, the subframe offset, and the
muting position.
8. The method of claim 7, wherein the data is transmitted based on
the muting position if the resource element in the subframe is to
be muted.
9. The method of claim 8, wherein the data is received on PDSCH
except for the muting position, and the data is not received on the
muting position.
10. The method of claim 8, wherein the data is mapped on PDSCH
except for the muting position, and the data is not mapped on the
muting position.
11. The method of claim 7, wherein the first information is
generated based on second information for neighbor cell.
12. The method of claim 11, wherein the second information
comprises channel state information reference signal (CSI-RS)
pattern for neighbor cell.
13. A terminal, comprising: a transceiver configured to transmit
and receive a signal; and a controller configured to receive first
information comprising a muting subframe interval, a subframe
offset, and a muting position of a resource element in a resource
block, check presence of a data in a subframe, determine the
resource element to be muted in the subframe based on the muting
subframe interval, the subframe offset, and the muting position, if
the data is present, and receive the data on a physical downlink
shared channel (PDSCH) based on the result of the determining
step.
14. The terminal of claim 13, wherein the data is received based on
the muting position if the resource element in the subframe is to
be muted.
15. The terminal of claim 14, wherein the data is received on PDSCH
except for the muting position, and the data is not received on the
muting position.
16. The terminal of claim 14, wherein the data is mapped on PDSCH
except for the muting position, and the data is not mapped on the
muting position.
17. The terminal of claim 13, wherein the first information is
generated based on second information for neighbor cell.
18. The terminal of claim 17, wherein the second information
comprises channel state information reference signal (CSI-RS)
pattern for neighbor cell.
19. A base station, comprising: a transceiver configured to
transmit and receive a signal; and a controller configured to
generate first information comprising a muting subframe interval, a
subframe offset, and a muting position of a resource element in a
resource block, transmit the first information to a terminal, and
transmit a data on a physical downlink shared channel (PDSCH) based
on the resource element to be muted in a subframe, wherein the
resource element is determined based on the muting subframe
interval, the subframe offset, and the muting position.
20. The method of claim 19, wherein the data is transmitted based
on the muting position if the resource element in the subframe is
to be muted.
21. The method of claim 20, wherein the data is received on PDSCH
except for the muting position, and the data is not received on the
muting position.
22. The method of claim 20, wherein the data is mapped on PDSCH
except for the muting position, and the data is not mapped on the
muting position.
23. The method of claim 19, wherein the first information is
generated based on second information for neighbor cell.
24. The method of claim 23, wherein the second information
comprises channel state information reference signal (CSI-RS)
pattern for neighbor cell.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a continuation application of U.S.
patent application Ser. No. 14/154,770, filed on Jan. 14, 2014,
which is a continuation of U.S. patent application Ser. No.
12/987,436, filed on Jan. 10, 2011, which issued as U.S. Pat. No.
8,634,363 on Jan. 21, 2014, and which claimed the benefit under 35
U.S.C. .sctn.119(a) of a Korean patent application filed on Jan.
12, 2010 and Feb. 11, 2010 in the Korean Intellectual Property
Office and assigned Serial number 10-10-2010-0002837 and
10-2010-0013005 respectively, the entire disclosure of which is
hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a wireless communication
system. More particularly, the present invention relates to a
method for processing a Channel State Information Reference Signal
(CSI-RS) in a wireless communication system based on a multiple
access scheme.
[0004] 2. Description of the Related Art
[0005] In 3.sup.rd Generation (3G) advanced wireless mobile
communication system standards, two types of reference signals are
specified, namely a Common Reference Signal (CRS) and a Dedicated
Reference Signal (DRS). CRS is referred to as either a
cell-specific RS or Common RS (CRS) in a 3G Partnership Project
(3GPP) Long Term Evolution (LTE) standard and is monitored by all
User Equipments (UEs) in a cell of a corresponding base station.
For multiple antenna transmission, reference signal patterns are
defined to distinguish between the antenna ports for channel
estimation and measurement. In an LTE system, a maximum of 4
antenna ports can be supported. DRS denotes a reference signal that
is separately transmitted from a CRS and listened to by a UE
indicated by the base station. In a 3GPP LTE-Advanced (LTE-A)
system, this reference signal is referred to as a UE-specific RS, a
DRS, or a Demodulation Reference Signal (DMRS) and is used for
supporting a data traffic channel transmission with non-codebook
based precoding at the base station.
[0006] In the LTE-A system, which is an advanced form of the LTE
system, a DeModulation Reference Signal (DM-RS) is transmitted for
supporting channel estimation with 8 layers in addition to the
aforementioned CRS and DRS.
[0007] FIG. 1 is diagram illustrating configurations of a radio
frame, a subframe, and a Physical Resource Block (PRB) for
transmitting CRS in an LTE system according to the related art.
[0008] Referring to FIG. 1, a radio frame is divided into 10
subframes, each having a length of 1 msec. This means that a radio
frame has a length of 10 msec and consists of 10 subframes as shown
in FIG. 1. In FIG. 1, reference number 110 denotes one of the
subframes constituting the radio frame. For each subframe, an
evolved Node B (eNB) performs transmission over the system
bandwidth in Orthogonal Frequency Division Multiple Access (OFDMA).
One subframe consists of a plurality of Physical Resource Blocks
(PRBs). One PRB consists of 12 subcarriers. For one subframe, the
subcarriers are arranged at a regular interval in the frequency
domain. In FIG. 1, reference number 120 denotes one of the PRBs
constituting the system bandwidth. In the LTE signal structure of
FIG. 1, a number of PRBs is determined depending on the system
bandwidth.
[0009] The PRB 120 is a time-frequency resource region as denoted
by reference number 130. As denoted by reference number 130 of FIG.
1, each PRB is a time-frequency resource region consisting of 12
subcarriers in the frequency domain and 14 OFDMA symbol durations
in the time domain. The resource unit defined by one subcarrier and
one OFDM symbol duration is referred to as a Resource Element (RE),
and one RE can carry one data symbol or reference signal
symbol.
[0010] The PRB 130 consists of 12 subcarriers and 14 OFDM symbol
durations. This means that a PRB 130 consists of a total of 168
REs. The first three OFDM symbol durations of the PRB 130 are
assigned as a control region in which the eNB uses a control
channel for transmitting control information with which the UE can
receive a traffic channel. Although the control region is defined
by the first three OFDM symbol durations, it can be configured with
the first one or two OFDM symbol durations depending on the eNB's
determination.
[0011] In FIG. 1, reference number 140 denotes a data RE for use in
transmitting traffic channel. Reference number 150 denotes a CRS RE
for use in transmitting a CRS for a UE's channel estimation and
measurement. Since the positions of the data RE and CRS RE are
known to the eNB and UE, the UE can receive the CRS and traffic
channel correctly in the PRB. Unless specifically stated otherwise,
all indexing starts from 0 in the following description. For
example, in FIG. 1, the 14 OFDM symbols constituting the PRB are
indexed from 0 to 13.
[0012] FIG. 2 is a diagram illustrating resources allocated for a
UE to report a channel quality measurement to an eNB in an LTE
system according to the related art.
[0013] Referring to FIG. 2, the UE measures a channel quality of
all the PRBs within the system band for the subframe 230 including
a plurality of PRBs. In order to measure the channel quality in
each PRB, the UE uses the CRS 220 transmitted by the eNB. Since the
CRS is transmitted at the same transmission power in all of the
PRBs, the UE can determine which PRB has relatively higher channel
quality by comparing the received signal strengths of the CRSs
received in respective PRBs. Also, it is possible to determine the
data rate which each PRB can support depending on the absolute
received signal strength. The channel quality information is mapped
in the form of channel feedback information and then reported to
the eNB using the uplink control channel as denoted by reference
number 240 of FIG. 2. Based on the channel feedback information
transmitted by the UE, the eNB performs downlink transmission in
the subframes 251, 252, 253, 254, and 255. The eNB can acquire the
information on the data rate, preferable precoding, and preferable
PRB supported by the UE based on the channel feedback information
transmitted by the UE and performs downlink scheduling and Adoptive
Modulation and Coding (AMC) based on the acquired information.
[0014] In FIG. 2, the eNB uses the channel feedback information 240
before the receipt of the next channel feedback information 260.
Although it is depicted that only one UE transmits the channel
feedback information in FIG. 2, a real world system is typically
designed such that a plurality of UEs can transmit the channel
feedback information simultaneously.
[0015] However, the method described above has a number of
problems. For example, in the LTE system, the UEs measure the
channel quality based on the CRS transmitted by the eNB. In case of
measuring the channel quality with a CRS as shown in FIG. 2, the
number of layers for the eNB to transmit with Multiple-Input
Multiple-Output (MIMO) technology is limited by the number of
antenna ports of a CRS. According to the standard, the LTE system
can support up to 4 antenna ports. Since more than four CRS antenna
ports are not supported, the MIMO transmission of the eNB is
limited to a maximum of four layers.
[0016] Another problem with CRS-based channel estimation and
measurement of the UEs is that the eNB must always transmit a CRS.
Accordingly, in order to support more than four antenna ports, an
additional CRS should be transmitted. This means that the limited
radio resource is excessively concentrated on transmitting a CRS
for channel estimation measurement, resulting in bandwidth
inefficiency.
SUMMARY OF THE INVENTION
[0017] An aspect of the present invention is to address at least
the above-mentioned problems and/or disadvantages and to provide at
least the advantages described below. Accordingly, an aspect of the
present invention is to provide a method for transmitting a Channel
State Information Reference Signal (CSI-RS) that is capable of
improving resource management efficiency of an evolved Node B (eNB)
and channel measurement efficiency of a User Equipment (UE) in a
Long Term Evolution Advanced (LTE-A) system.
[0018] Another aspect of the present invention is to provide a
method for transmitting a CSI-RS that is capable of improving radio
resource management efficiency in view of each eNB and separating
the CSI-RSs transmitted in the cells of corresponding eNBs in the
time domain and the frequency domain in view of multiple eNBs.
[0019] In accordance with an aspect of the present invention, a
method for transmitting a CSI-RS in an Orthogonal Frequency
Division Multiple Access (OFDMA) system is provided. The method
includes determining a CSI-RS pattern type based on a Physical
Resource Block (PRB) index of a subframe, assigning, when the
subframe is supposed to carry the CSI-RS, CSI-RSs of first to
N.sup.th antenna ports to first to N.sup.th Orthogonal Frequency
Division Multiplexing (OFDM) symbols of a PRB based on the CSI-RS
pattern type, and transmitting the subframe including the PRB in
which CSI-RSs of the first to N.sup.th antennas a mapped, wherein
the first to N.sup.th CSI-RS pattern types map CSI-RSs of the first
to N.sup.th antenna ports to the first to N.sup.th OFDM symbols of
the PRB in an alternate manner.
[0020] In accordance with another aspect of the present invention,
a method for transmitting a CSI-RS in an OFDMA system is provided.
The method includes determining cells in a Coordinated Multi Point
(CoMP) set and sharing information related to CoMP CSI-RS
transmission among the cells, determining, when transmitting a
subframe, whether the subframe is supposed to carry a CSI-RS,
determining, when the subframe is supposed to carry CSI-RS, whether
the CSI-RS is a CoMP CSI-RS, and transmitting, when the CSI-RS is a
CoMP CSI-RS, the CoMP CSI-RS and, when the CSI-RS is not CoMP
CSI-RS, a non-CoMP CSI-RS, wherein the CoMP CSI-RS is the CSI-RS
transmitted by a plurality of cells for a User Equipment (UE) to
measure downlink channels of the cells, and the CoMP set is a set
of the cells participating in cooperation for transmitting the CoMP
CSI-RS.
[0021] In accordance with still another aspect of the present
invention, a method for transmitting a Channel State
Information-Reference Signal (CSI-RS) in an Orthogonal Frequency
Division Multiple Access (OFDMA) system is provided. The method
includes sharing information on CSI-RS patterns of adjacent cells
and a number of CSI-RS antenna ports, determining, when a subframe
is transmitted, whether the subframe is supposed to carry a CSI-RS
by analyzing the CSI-RS patterns, transmitting, when CSI-RS
transmission time is up, the subframe carrying the CSI-RS according
to the CSI-RS pattern, and transmitting, at a CSI-RS transmission
time of one of the adjacent cells, a subframe in which a number of
Resource Elements (REs) corresponding to the number of CSI-RS
antenna ports of the adjacent cells are muted.
[0022] In accordance with yet another aspect of the present
invention, a method for wireless communication by a terminal is
provided. The method includes receiving first information
comprising a muting subframe interval, a subframe offset, and a
muting position of a resource element in a resource block, checking
presence of a data in a subframe, determining the resource element
to be muted in the subframe based on the muting subframe interval,
the subframe offset, and the muting position, if the data is
present, and receiving the data on a physical downlink shared
channel (PDSCH) based on the result of the determining step.
[0023] In accordance with still another aspect of the present
invention, a method for wireless communication by a base station is
provided. The method includes generating first information
comprising a muting subframe interval, a subframe offset, and a
muting position of a resource element in a resource block,
transmitting the first information to a terminal, and transmitting
a data on a physical downlink shared channel (PDSCH) based on the
resource element to be muted in a subframe, wherein the resource
element is determined based on the muting subframe interval, the
subframe offset, and the muting position.
[0024] In accordance with yet another aspect of the present
invention, a terminal is provided. The terminal includes a
transceiver configured to transmit and receive a signal, and a
controller configured to receive first information comprising a
muting subframe interval, a subframe offset, and a muting position
of a resource element in a resource block, check presence of a data
in a subframe, determine the resource element to be muted in the
subframe based on the muting subframe interval, the subframe
offset, and the muting position, if the data is present, and
receive the data on a physical downlink shared channel (PDSCH)
based on the result of the determining step.
[0025] In accordance with still another aspect of the present
invention, a base station is provided. The base station includes a
transceiver configured to transmit and receive a signal, and a
controller configured to generate first information comprising a
muting subframe interval, a subframe offset, and a muting position
of a resource element in a resource block, transmit the first
information to a terminal, and transmit a data on a physical
downlink shared channel (PDSCH) based on the resource element to be
muted in a subframe, wherein the resource element is determined
based on the muting subframe interval, the subframe offset, and the
muting position.
[0026] Other aspects, advantages, and salient features of the
invention will become apparent to those skilled in the art from the
following detailed description, which, taken in conjunction with
the annexed drawings, discloses exemplary embodiments of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The above and other aspects, features, and advantages of
certain exemplary embodiments of the present invention will be more
apparent from the following description taken in conjunction with
the accompanying drawings, in which:
[0028] FIG. 1 is diagram illustrating configurations of a radio
frame, a subframe, and a Physical Resource Block (PRB) for
transmitting a Common Reference Signal (CRS) in a Long Term
Evolution (LTE) system according to the related art;
[0029] FIG. 2 is a diagram illustrating resources allocated for a
User Equipment (UE) to report a channel quality measurement to an
evolved Node B (eNB) in an LTE system according to the related
art;
[0030] FIG. 3 is a diagram illustrating a Channel State Information
Reference Signal (CSI-RS) transmission of an eNB in an LTE-Advanced
(LTE-A) system according to an exemplary embodiment of the present
invention;
[0031] FIG. 4 is a diagram illustrating locations of the CSI-RS in
a time frequency grid for an LTE-A system according to an exemplary
embodiment of the present invention;
[0032] FIG. 5 is a diagram illustrating a PRB having Resource
Elements (REs) allocated for transmitting CSI-RSs in an LTE-A
system according to an exemplary embodiment of the present
invention;
[0033] FIG. 6 is a diagram illustrating a PRB having REs allocated
for transmitting CSI-RSs alternately in an LTE-A system according
to an exemplary embodiment of the present invention;
[0034] FIG. 7 is a diagram illustrating a principle of transmitting
CSI-RSs with CSI-RS pattern type A and CSI-RS pattern type B of
FIG. 6 in a system bandwidth according to an exemplary embodiment
of the present invention;
[0035] FIG. 8 is a diagram illustrating a principle of transmitting
CSI-RSs of antenna ports alternately in different PRBs with four
CSI-RS patterns according to an exemplary embodiment of the present
invention;
[0036] FIG. 9 is a diagram illustrating CSI-RS patterns designed to
be assigned to a plurality of cells in a mobile communication
system according to an exemplary embodiment of the present
invention;
[0037] FIG. 10 is a diagram illustrating a cellular layout of a
mobile communication system adopting the CSI-RS patterns as defined
in FIG. 9 according to an exemplary embodiment of the present
invention;
[0038] FIG. 11 is a diagram illustrating CSI-RS patterns designed
to be assigned to a plurality of cells in a mobile communication
system according to an exemplary embodiment of the present
invention;
[0039] FIG. 12 is a diagram illustrating a PRB in which three
CSI-RS regions are defined according to an exemplary embodiment of
the present invention;
[0040] FIG. 13 is a diagram illustrating a principle for
transmitting CSI-RSs in a cellular environment including various
types of cells according to an exemplary embodiment of the present
invention;
[0041] FIGS. 14a, 14b, and 14c are diagrams illustrating CSI-RS
patterns designed to be assigned to a plurality of cells in a
mobile communication system according to an exemplary embodiment of
the present invention;
[0042] FIG. 15 is a diagram illustrating a principle for
transmitting the CSI-RSs in a plurality of cells without
interference according to an exemplary embodiment of the present
invention;
[0043] FIG. 16 is a diagram illustrating a principle for
transmitting CSI-RSs of multiple cells with different CSI-RS
patterns in a subframe according to an exemplary embodiment of the
present invention;
[0044] FIG. 17 is a diagram illustrating a principle for
transmitting Coordinated Multi Point (CoMP) CSI-RS and non-CoMP
CSI-RS in a cell of a mobile communication system according to an
exemplary embodiment of the present invention;
[0045] FIG. 18 is a diagram illustrating a principle for
transmitting CoMP CSI-RS and non-CoMP CSI-RS in a cell of a mobile
communication system according to an exemplary embodiment of the
present invention;
[0046] FIG. 19 is a diagram illustrating a principle for
transmitting CoMP CSI-RS and non-CoMP CSI-RS in a mobile
communication system according to an exemplary embodiment of the
present invention;
[0047] FIG. 20 is a diagram illustrating a principle of
transmitting CSI-RS with a muting scheme in a mobile communication
system according to an exemplary embodiment of the present
invention;
[0048] FIG. 21 is a diagram illustrating a principle of
transmitting CSI-RS with a muting scheme in a mobile communication
according to an exemplary embodiment of the present invention;
[0049] FIG. 22 is a diagram illustrating a principle for applying a
muting scheme to a case where multiple cells transmit CSI-RSs with
different subframe offsets according to an exemplary embodiment of
the present invention;
[0050] FIG. 23 is a diagram illustrating a principle for applying a
muting scheme applied to a case where two cells transmit CSI-RS in
some PRBs according to an exemplary embodiment of the present
invention;
[0051] FIG. 24 is a flowchart illustrating a method for a UE to
receive CSI-RS in a mobile communication system operating with a
plurality of CSI-RS patterns depicted in FIG. 9 or FIG. 11
according to an exemplary embodiment of the present invention;
[0052] FIG. 25 is a flowchart illustrating a method for a UE to
receive CSI-RS in a mobile communication system operating with a
plurality of CSI-RS patterns depicted in FIG. 9 or FIG. 11 with a
muting scheme according to an exemplary embodiment of the present
invention;
[0053] FIG. 26 is a diagram illustrating a method for an eNB to
transmit CSI-RS with a muting scheme in a mobile communication
system according to an exemplary embodiment of the present
invention;
[0054] FIG. 27 is a diagram illustrating a method for an eNB to
transmit non-CoMP CSI-RS and CoMP CSI-RS in a mobile communication
according to an exemplary embodiment of the present invention;
and
[0055] FIG. 28 is a flowchart illustrating a method for a UE to
receive non-CoMP CSI-RS and CoMP CSI-RS in a mobile communication
according to an exemplary embodiment of the present invention.
[0056] Throughout the drawings, it should be noted that like
reference numbers are used to depict the same or similar elements,
features, and structures.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0057] The following description with reference to the accompanying
drawings is provided to assist in a comprehensive understanding of
exemplary embodiments of the invention as defined by the claims and
their equivalents. It includes various specific details to assist
in that understanding but these are to be regarded as merely
exemplary. Accordingly, those of ordinary skill in the art will
recognize that various changes and modifications of the embodiments
described herein can be made without departing from the scope and
spirit of the invention. In addition, descriptions of well-known
functions and constructions may be omitted for clarity and
conciseness.
[0058] The terms and words used in the following description and
claims are not limited to their bibliographical meanings, but, are
merely used by the inventor to enable a clear and consistent
understanding of the invention. Accordingly, it should be apparent
to those skilled in the art that the following description of
exemplary embodiments of the present invention is provided for
illustration purpose only and not for the purpose of limiting the
invention as defined by the appended claims and their
equivalents.
[0059] It is to be understood that the singular forms "a," "an,"
and "the" include plural referents unless the context clearly
dictates otherwise. Thus, for example, reference to "a component
surface" includes reference to one or more of such surfaces.
[0060] Exemplary embodiments of the present invention relate to a
method for transmitting a Channel State Information Reference
Signal (CSI-RS) and controlling CSI-RS transmission in which a User
Equipment (UE) measures channel quality based on the CSI-RS
transmitted by an evolved Node B (eNB) in a mobile communication
system based on a multiple access scheme such as Orthogonal
Frequency Division Multiple Access (OFDMA) using multiple carriers.
That is, exemplary embodiments of the present invention propose a
method for transmitting/receiving reference signals and managing
reference signal transmission efficiently in multiple cells.
[0061] The mobile communication system has been developed as a
high-speed, high-quality packet data communication system for
providing various multimedia services as well as basic voice
service. To this end, the standardization organizations such as the
3.sup.rd Generation Partnership Project (3GPP), the 3GPP2, and the
Institute of Electrical and Electronics Engineers (IEEE) are
standardizing next generation mobile communication systems adopting
multi-carrier-based multiple access schemes. Recently, various
mobile communication standards such as 3GPP Long Term Evolution
(LTE), 3GPP2 Ultra Mobile Broadband (UMB), and IEEE 802.16m have
been developed to support high-speed, high-quality wireless packet
data transmission service based on the multi-carrier multiple
access scheme.
[0062] The advanced 3.sup.rd Generation (3G) mobile communication
systems such as LTE, UMB, and 802.16m are operating based on the
multi-carrier multiple access schemes and adopt various techniques
such as Multiple Input Multiple Output (MIMO), beamforming,
Adaptive Modulation and Coding (AMC), and channel sensitive
scheduling in order to improve transmission efficiency. These
transmission techniques allow for the concentration of transmission
power of multiple antennas or the adjustment of a data rate and
improve system throughput with enhanced transmission efficiency by
transmitting data selectively to the user having good channel
quality. Most of these transmission techniques are based on the
channel state information between the Base Station (BS) (e.g., an
eNB) and a terminal (e.g., a UE or a Mobile Station (MS)), and
CSI-RS is used as the channel state information. The eNB can be a
downlink transmission and uplink reception device installed at a
location, and an eNB can operate multiple cells. A mobile
communication system includes a plurality of eNBs that are
geographically distributed and each eNB manages
transmission/reception in multiple cells.
[0063] Typically, the mobile communication system operates with
limited time, frequency, and transmission power resources.
Accordingly, allocating large amounts of resources for reference
signals decreases the resources allocated for traffic channel
transmission, resulting in a reduction in the amount of
transmission. In this case, although the channel measurement and
estimation performance is improved, the reduction of the amount of
transmission data causes deterioration of overall system
throughput. There is therefore a need to allocating resources
efficiently for transmission of reference signals and traffic
channel in order to optimize the performance in view of overall
system throughput.
[0064] The reference signal is used to measure the channel state
between the base station and the UE such as a signal strength and
distortion per channel, interference, and Gaussian noise and
determine the modulation and decoding of the received data symbol
based on the measurements. The receiver measures the received
strength of the reference signal transmitted by the transmitter at
a promised transmission power so as to determine the radio channel
state with the transmitter. The radio channel state is used to
determine the data rate which the receiver requests from the
transmitter.
[0065] The advanced 3G mobile communication standard such as 3GPP
LTE-Advanced (LTE-A) or IEEE 802.16m adopts Orthogonal Frequency
Division Multiplexing/Orthogonal Frequency Multiple Access
(OFDM/OFDMA) as a multi-carrier multiple access transmission
scheme. In the mobile communication system based on the
multi-carrier multiple access scheme, the channel estimation and
measurement performance can be influenced by the number of symbols
in the time domain and the number of subcarriers in the frequency
domain carrying the reference signals. Also, the channel estimation
and measurement performance is influenced by the power allocated
for transmission of reference signals. The more the time,
frequency, and power are allocated, the more the channel estimation
and measurement performance improves, and this results in the
improvement of data symbol demodulation and decoding performance
and channel state measurement accuracy.
[0066] However, since the resources such as the time, frequency,
and transmission power are limited in a typical mobile
communication system, an excessive increase of resource allocation
for a reference signal causes a decrease of the resources for
transmission of a data signal. For this reason, the resource
allocation for the reference signals should be determined in
consideration of system throughput.
[0067] An exemplary embodiment of the present invention proposes a
method for transmitting/receiving reference signals for channel
quality measurement of radio channels and managing the reference
signal transmission efficiently in multiple cells.
[0068] FIG. 3 is a diagram illustrating CSI-RS transmission of an
eNB in an LTE-A system according to an exemplary embodiment of the
present invention.
[0069] Referring to FIG. 3, in an LTE-A system downlink
transmission is performed in a unit of 1 msec in the time domain
and 1 Physical Resource Block (PRB) in the frequency domain. Here,
a PRB consists of 12 subcarriers. The 1 msec time duration consists
of 14 OFDM symbols. 12 subcarriers per PRB and 14 OFDM symbols per
1 msec are implemented when the LTE or LTE-A system uses the
subcarriers spaced at 15 KHz intervals and the normal cyclic
prefix. However, the LTE or LTE-A system also may use the
subcarriers spaced at 7.5 KHz intervals and the extended cyclic
prefix.
[0070] In FIG. 3, the eNB transmits subframes 340 to 351. In this
case, the subframes 340, 345, and 350 among the subframes 340 to
351 are used to carry the CSI-RSs. That is, the CSI-RS is
transmitted at an interval of 5 msec or 5 subframes. If the
subframe 340 carries the CSI-RS, this means the CSI-RS is
transmitted in one or more PRBs of the subframe 340. In FIG. 3,
reference number 335 denotes the PRB carrying the CSI-RS among a
plurality of PRBs constituting the subframe 340. In the PRB 335,
individual CSI-RSs 331, 332, 333, and 334 are transmitted on the
corresponding antenna ports. That is, the CSI-RS 331 is transmitted
on the antenna ports 0 and 1, whereas the CSI-RS 332 on the antenna
ports 2 and 3.
[0071] In FIG. 3, reference number 336 denotes the PRB which does
not carry CSI-RS. The PRB which does not carry the CSI-RS is
transmitted in the form denoted by reference number 336 and
compared to the PRB 335 carrying the CSI-RS.
[0072] The LTE-A system differs from the LTE system in that the
LTE-A UEs performs channel measurement using CSI-RSs 331, 332, 333,
and 334 rather than CRS 320.
[0073] In order to design an efficient CSI-RS transmission scheme,
it is the locations in which the CSI-RSs are transmitted in the
time-frequency grid that should be determined.
[0074] FIG. 4 is a diagram illustrating locations of a CSI-RS in a
time frequency grid for an LTE-A system according to an exemplary
embodiment of the present invention.
[0075] As shown in FIG. 4, the PRB includes various types of
Resource Elements (REs). Here, the RE is defined by one subcarrier
in the frequency domain and one OFDM symbol duration in the time
domain and is the smallest unit of resource for transmission in the
LTE and LTE-A systems. In FIG. 4, each of the squares indicates an
RE. Here, there are a total of 12.times.14 squares and thus a total
of 12.times.14 REs.
[0076] Referring to FIG. 4, the first three OFDM symbol durations
(i.e., 0.sup.th to 2.sup.nd OFDM symbols) are the control region in
which only the control signal and CRSs (e.g., CRS 410) are
transmitted. The control region is monitored by the LTE UEs
operating in the LTE-A system and as a consequence there is no need
to carry the CSI-RSs. From the 3.sup.rd to the last OFDM symbol
durations (i.e., 3.sup.rd to 13.sup.th OFDM symbols) are a data
region in which a traffic channel signal, an LTE UE-dedicated
reference signal for channel estimation (or in LTE-A, a UE-specific
RS), and a CRS can be transmitted. Since the CSI-RS cannot be
transmitted in the control region, it must be transmitted in the
data region (such as at RE 430). Although the CSI-RS can be
transmitted in the data region, it should be avoided to locate it
in the REs at which the CSI-RS can influence the conventional LTE
transmission operation. The representative REs that should not be
assigned for the CSI-RS are the REs in the 4.sup.th, 7.sup.th,
8.sup.th, and 11.sup.th OFDM symbols, the REs reserved for the LTE
UE-specific reference signal as denoted by reference number 420 of
FIG. 4, and the REs reserved for the LTE-A UE-specific reference
signal as denoted by reference number 440 of FIG. 4.
[0077] FIG. 5 is a diagram illustrating a PRB having REs allocated
for transmitting CSI-RSs in an LTE-A system according to an
exemplary embodiment of the present invention.
[0078] Referring to FIG. 5, the CSI-RSs positioned at the REs 510,
520, 530, 540, 550, and 560 are the only CSI-RS in each OFDM
symbols of the PRB. This means that, in the OFDM symbol carrying
the CSI-RS, only one CSI-RS can be transmitted on an antenna port.
Transmitting one CSI-RS on the single antenna port in an OFDM
symbol is inefficient in view of transmission power management of
the eNB. If transmitting only one CSI-RS for a single antenna port
in one OFDM symbol as shown in FIG. 5, even when the eNB is
configured with a plurality of antenna ports, transmission power at
the remaining antenna ports is wasted. In an exemplary case where
the CSI-RSs for the four antenna ports are transmitted in the PRB
as shown in FIG. 5, the OFDM symbol having the RE 510 carries the
CSI-RS for a specific antenna port. The problem is that, since the
CSI-RS for only one antenna port is transmitted, the transmission
power allocated for the other three antenna ports are not
utilized.
[0079] The problem also occurs when the CSI-RSs for the two antenna
ports are transmitted in an OFDM symbol. The CSI-RSs 540 and 570
are transmitted in the same OFDM symbol for the respective antenna
ports. The problem is that, when the number of antenna ports of the
eNB is greater than 2, the transmission power at the antenna ports
except for the antenna ports for which the CSI-RSs 540 and 570 are
transmitted are not utilized as aforementioned.
[0080] One approach to use the transmission powers of all the
antenna ports of the eNB is to transmit the CSI-RSs for all the
antenna ports in one OFDM symbol. However, this approach has a
limitation in consideration that one PRB consists of only 12
subcarriers, the maximum number of antenna ports is 8, and some REs
cannot be allocated for CSI-RS.
[0081] An exemplary embodiment of the present invention proposes a
method for utilizing the transmission powers of all the antenna
ports of the eNB in which the eNB applies the CSI-RSs of antenna
ports to different PRBs alternately.
[0082] FIG. 6 is a diagram illustrating a PRB having REs allocated
for transmitting CSI-RSs alternately in an LTE-A system according
to an exemplary embodiment of the present invention.
[0083] Referring to FIG. 6, the eNB transmits the CSI-RS in the
first OFDM symbol and the second OFDM symbol. In order to use the
transmission power of all the antenna ports, the eNB transmits the
CSI-RS in a first CSI-RS pattern for the some PRBs and in a second
CSI-RS pattern for other PRBs. In the first CSI-RS pattern, the
CSI-RSs for the first set of antenna ports are transmitted in the
first one of the two contiguous OFDM symbols, and the CSI-RSs for
the second set of antenna ports are transmitted in the second one
of the two contiguous OFDM symbols; and in the second CSI pattern,
the CSI-RSs for the second set of antenna ports are transmitted in
the preceding-OFDM symbol, and the CSI-RSs for the first set of
antenna ports are transmitted in the following-OFDM symbol.
[0084] In FIG. 6, the first one OFDM symbol can be the 9th OFDM
symbol, and the second one OFDM symbol can be the 10th OFDM symbol
in the PRB. The first CSI-RS pattern can be the CSI-RS pattern type
A 610, and the second CSI-RS pattern can be the CSI-RS pattern type
B 620.
[0085] In FIG. 6, it is assumed that the eNB transmits the CSI-RSs
in the 9.sup.th and 10.sup.th OFDM symbols. At this time, the eNB
can transmit the CSI-RS in the CSI-RS pattern type A 610 and the
CSI-RS pattern type B 620. In the CSI-RS pattern type A 610, the
CSI-RSs for the antenna ports 4, 5, 6, and 7 are transmitted in the
9.sup.th OFDM symbol and the CSI-RSs for the antenna ports 0, 1, 2,
and 3 are transmitted in the 10.sup.th OFDM symbol. Whereas, in the
CSI-RS pattern type B 620, the CSI-RSs for the antenna ports 0, 1,
2, and 3 are transmitted in the 9.sup.th OFDM symbol, and the
CSI-RSs for the antenna ports 4, 5, 6, and 7 are transmitted in the
10.sup.th OFDM symbol. The eNB configures the half of the PRBs
carrying the CSI-RSs in the CSI-RS pattern type A 610 and the other
half of the PRBs carrying the CSI-RSs in the CSI-RS pattern type B
620 within the system bandwidth.
[0086] Defining a plurality of CSI-RSs and transmitting the CSI-RSs
in the PRBs at an identical rate within the system bandwidth has
the following advantages. First, a number of CSI-RSs transmitted in
the OFDM symbol is identical with that of the antenna ports.
Second, the CSI-RSs can be transmitted in multiple OFDM
symbols.
[0087] The first advantage can be fulfilled when, although the
CSI-RSs for different antenna ports are transmitted in the same
OFDM symbol with the CSI-RS pattern type A and CSI-RS pattern type
B, the two patterns are used simultaneously such that the CSI-RSs
for all the antenna ports exist in a single OFDM symbol duration.
Assuming an exemplary case of the system bandwidth including two
PRBs in which a first one PRB carries the CSI-RS in the CSI-RS
pattern type A and the second one PRB carries the CSI-RS in the
CSI-RS pattern type B, the CSI-RSs for all the antenna ports are
transmitted in each of the 9.sup.th and 10.sup.th OFDM symbols. In
a case that the CSI-RSs for all the antenna ports are transmitted
in a single OFDM symbol, it is possible to use the transmission
powers of all antenna ports. The second advantage can be fulfilled
with a high degree of freedom for determining the transmission
locations of the CSI-RSs by using multiple OFDM symbols for CSI-RS
transmission. Assuming that CSI-RSs for 8 antenna ports are
transmitted in a single OFDM symbol, the CSI-RSs cannot be
transmitted in the 5.sup.th, 6.sup.th, 12.sup.th, and 13.sup.th
OFDM symbols since there are only 6 REs in each of the 5.sup.th,
6.sup.th, 12.sup.th, and 13.sup.th OFDM symbols.
[0088] FIG. 7 is a diagram illustrating a principle of transmitting
CSI-RSs with a CSI-RS pattern type A and a CSI-RS pattern type B of
FIG. 6 in a system bandwidth according to an exemplary embodiment
of the present invention.
[0089] Referring to FIG. 7, the part 710 shows an exemplary CSI-RS
pattern determination scheme in which whether to use the CSI-RS
pattern type A 610 or the CSI-RS pattern type B 620 is determined
based on the PRB index, i.e., based on whether the PRB index is an
odd number or an even number. In the part 710 of FIG. 7, the PRBs
of which indices are even numbers (i.e., 0, 2, 4, 6, . . . ) are
determined to use the CSI-RS pattern type A 610 for transmission of
the CSI-RSs, and the PRBs of which indices are odd numbers (i.e.,
1, 3, 5, 7, . . . ) are determined to use the CSI-RS pattern type B
620 for the transmission of the CSI-RSs. The part 720 shows another
exemplary CSI-RS pattern determination scheme in which whether to
use the CSI-RS pattern type A 610 or the CSI-RS pattern type B 620
is determined based on whether the PRB index has a value less than
half of the maximum value (i.e., K/2, where K is the maximum value
of the PRB index). In the part 720 of FIG. 7, the PRBs of which
indices are less than half of the maximum value of PRB index (i.e.,
PRB 0 to PRB 2) are determined to use the CSI-RS pattern type A
610, and the PRBs of which indices are greater than half of the
maximum value of PRB index (i.e., PRB 3 to PRB 5) are determined to
use the CSI-RS pattern type B 620.
[0090] Although FIG. 7 is depicted under the assumption that two
CSI-RS patterns are used for transmission of CSI-RSs, the number of
CSI-RS patterns can be generalized by N as follows. Here, assume
that K PRBs exist in the system bandwidth (i.e., maximum value of
PRB index is K).
[0091] Solution 1: if PRB index is i, use (i mod N).sup.th CSI-RS
pattern type.
[0092] Solution 2: if PRB index is i, use .left
brkt-bot.i/(K/N).right brkt-bot..sup.th CSI-RS pattern type.
[0093] Solutions 1 and 2 are of the case where the CSI-RSs are
transmitted in all the PRBs, but can be applied to the cases where
CSI-RSs are not transmitted in all the PRBs similarly. In an
exemplary case where the CSI-RSs are transmitted every Lth PRB
(e.g., every 5th PRB, L=5), the solutions 1 and 2 can be applied in
identical manner. In a case where the CSI-RSs are transmitted in an
Lth PRB at an identical interval in the frequency domain, the PRB
for transmitting the CSI-RSs can be determined based on the PRB
index according to Table 1.
TABLE-US-00001 TABLE 1 If PRB index i If (i+offset) mod L = 0, PRB
is transmitting CSI-RS If (i+offset) mod L .noteq. 0, PRB is not
transmitting CSI-RS
[0094] In Table 1, the offset value is a variable for determining
the location in an interval when to determine the PRB is for
transmitting a CSI-RS at a regular interval. Even when the CSI-RSs
are transmitted in the PRB determined as above, the solutions 1 and
2 can be applied in the same manner as the following solutions 3
and 4.
[0095] Solution 3: if PRB index is i and the PRB carries CSI-RSs,
use (.left brkt-bot.i/L.right brkt-bot.mod N) or ((i-offset)/L) mod
N.sup.th CSI-RS pattern type.
[0096] Solution 4: if PRB index is i and the PRB carries CSI-RSs,
use (.left brkt-bot.i/(K/N).right brkt-bot..sup.th CSI-RS pattern
type.
[0097] Such a CSI-RS transmission method can be informed to the UE
in the LTE (or LTE-A) system through signaling. That is, in the LTE
(or LTE-A) system, the UE can be notified of one of the first to
fourth solutions and the CSI-RS pattern type and then transmit the
CSI-RSs to the UE in the CSI-RS pattern type determined by the PRB
index at the CSI-RS transmission timing.
[0098] FIG. 8 is a diagram illustrating a principle of transmitting
CSI-RSs of antenna ports alternately in different PRBs with four
CSI-RS patterns according to an exemplary embodiment of the present
invention.
[0099] Referring to FIG. 8, each CSI-RS pattern is designed for
CSI-RSs transmitted in four OFDM symbols. In this case, four CSI-RS
pattern types (i.e., type A to type D) are defined as shown in FIG.
8 such that the CSI-RSs of all antenna ports can be transmitted in
a single OFDM symbol. How to allocate resources for the CSI-RSs in
multiple PRBs within the system bandwidth can follow the CSI-RS
pattern type determination method using the PRB indices as
described with reference to FIG. 7.
[0100] The CSI-RS patterns used for utilizing the transmission
power of all antenna ports as shown in FIGS. 6 and 8 are
characterized by the cyclic rotation of the antenna ports
corresponding to the CSI-RSs to be transmitted. In the case of FIG.
8, it is observed that the CSI-RSs of the antenna ports that are
transmitted in the 5.sup.th, 6.sup.th, 9.sup.th, and 10.sup.th OFDM
symbols are cyclically rotating in the time domain. This cyclic
rotation feature is used to define the CSI-RS pattern for use of
the transmission power of all antenna ports.
[0101] When designing the CSI-RS transmission, it should be taken
into account which radio resources are allocated by multiple eNBs
for CSI-RS transmission and for the CSI-RSs of multiple cells of
one eNB. CSI-RS is the signal for the UEs to support LTE-A
operations. Accordingly, in order to measure the radio channel
state more accurately, the eNB may transmit the CSI-RS, for each
cell, at a higher power level than that for the data signal
transmission. Setting the transmission power for the CSI-RS
transmission higher than that for the data signal transmission
means that the transmission power of the RE carrying a CSI-RS is
higher than that of the RE carrying data. In a case where the
CSI-RS RE is allocated the transmission power higher than that of
the data RE and different cells transmit the CSI-RS at the same RE
positions, the CSI-RSs transmitted by the different cells are
likely to interfere with each other. In this case, even when the
CSI-RS is at a relatively higher transmission power, the positive
effect of the channel measurement is diminished. Accordingly, there
is a need for a method to perform the channel measurement
accurately even with the relatively higher CSI-RS transmission
power. In order to address this problem, an exemplary embodiment of
the present invention proposes using individual CSI-RS patterns
that can be allocated to multiple cells. In an exemplary embodiment
of the present invention, a plurality of CSI-RSs is defined to be
assigned to the cells constituting the cellular system.
[0102] FIG. 9 is a diagram illustrating CSI-RS patterns designed to
be assigned to a plurality of cells in a mobile communication
system according to an exemplary embodiment of the present
invention.
[0103] Referring to FIG. 9, a plurality of CSI-RS patterns designed
to carry the CSI-RSs are shown. The CSI-RS patterns are allocated
such that the cells use the resources at the different times and
frequencies in order to avoid the overlapping the CSI-RS
transmission positions of the cells by as much as possible. The
CSI-RS patterns are allocated to transmit the CSI-RS of multiple
antenna ports in the same OFDM symbol and use different resources
with different OFDM symbols in the time domain. In a case of using
the same OFDM symbol, the CSI-RS patterns are designed such that
CSI-RSs of the antenna ports are arranged alternately to occupy
different resources in the frequency domain within the system
bandwidth.
[0104] The 6 CSI-RS patterns defined in FIG. 9 are allocated to
different cells in a distributed manner to avoid the overlap of the
locations for the CSI-RS transmission by as much as possible. In
FIG. 9, CSI-RS pattern 0 is composed of the REs different from
those of the CSI-RS patterns 2, 3, 4, and 5. For this reason, even
though the transmission powers of the CSI-RSs transmitted by other
cells with the CSI-RS patterns 2, 3, 4, and 5 are increased, the UE
can measure the channel with the CSI-RS transmitted by the cell
using the CSI-RS pattern 0 without additional interference. The
reason why the CSI-RS transmitted in the CSI-RS pattern 0 is not
interfered by the CSI-RSs transmitted in the CSI-RS patterns 2, 3,
4, and 5 is because different time and frequency resources are
used, and this is achieved regardless of the number of CSI-RS
antenna ports. In an exemplary case where CSI-RSs are transmitted
for two antenna ports, it is possible to prevent the interference
from occurring by transmitting the CSI-RSs at the positions for the
antenna ports 0 and 1.
[0105] In FIG. 9, the CSI-RS pattern 0 and the CSI-RS pattern 1 may
cause interference with each other. In a case where the number of
antennas of which CSI-RSs are transmitted in the CSI-RS pattern 0
is equal to or less than 4 and the number of antennas of which
CSI-RSs are transmitted in the CSI-RS pattern 1 is equal to or less
than 4, the CSI-RSs transmitted in the CSI-RS patterns 0 and 1 do
not interfere with each other. Interference occurs either when the
number of antennas of which CSI-RSs are transmitted in the CSI-RS
pattern 0 is greater than 4 or the number of antennas of which
CSI-RSs are transmitted in the CSI-RS pattern 1 is greater than 4.
The reason why the interference occurs in some limited cases is
because the CSI-RS patterns are designed such that the CSI-RSs
alternate in the same OFDM symbol. That is, the order of the
antenna ports of which CSI-RSs are transmitted in the CSI-RS
pattern 0 is 0, 4, 1, 5, 2, 6, 3, and 7, whereas the order of the
antenna ports of which CSI-RSs are transmitted in the CSI-RS
pattern 1 is 4, 0, 5, 1, 6, 2, 7, and 3 in the same OFDM
symbol.
[0106] The principle described with regard to the CSI-RS pattern 0
is applied to the CSI-RS patterns 1, 2, 3, 4, and 5 identically.
The CSI-RS patterns defined in an exemplary embodiment of the
present invention as shown in FIG. 9 can be applied to the mobile
communication system composed of a plurality of cells.
[0107] FIG. 10 is a diagram illustrating a cellular layout of a
mobile communication system adopting CSI-RS patterns as defined in
FIG. 9 according to an exemplary embodiment of the present
invention.
[0108] Referring to FIG. 10, the mobile communication system is
composed of 7 hexagonal service areas of individual eNBs, and each
service area is divided into three cells. In the three cells of
each service area, the CSI-RSs are transmitted in different CSI-RS
patterns. For example, the eNB having three cells 1010, 1020, and
1030 transmits a CSI-RS in the CSI-RS pattern 0 within cell 1010,
in the CSI-RS pattern 4 within cell 1020, and in the CSI-RS pattern
3 within cell 1030.
[0109] In order to assign the CSI-RS patterns as defined in FIG. 9
to the cells of the mobile communication system as shown in FIG.
10, an exemplary embodiment of the present invention proposes the
use of cell IDs. According to an exemplary embodiment of the
present invention, if the cell ID is NCell_ID, the CSI-RS pattern
ID is determined by Equation (1).
CSI_RS Pattern ID=NCell_ID mod 6 Equation (1)
[0110] With Equation (1), it is possible to assign the six CSI-RS
patterns of FIG. 9 according to the cell IDs assigned to the
individual cells without separate signaling between the eNB and
UEs. In addition to the method using Equation (1), it is possible
to determine which CSI-RS patterns are to be assigned to the cells
through higher layer signaling and notify the CSI-RS patterns to
the UEs in the form of control information.
[0111] One approach to assign the CSI-RS pattern is to classify the
CSI-RS patterns into more than two groups and use the CSI-RSs of
each group for different purposes. For example, the six CSI-RS
patterns of FIG. 9 can be grouped into two groups, namely group A
and group B. In this case, it is assumed that group A includes
CSI-RS pattern 0, CSI-RS pattern 2, and CSI-RS pattern 4, and group
B includes CSI-RS pattern 1, CSI-RS pattern 3, and CSI-RS pattern
5. If the six CSI-RS patterns of FIG. 9 are grouped into group A
and group B in this manner, it is possible to avoid the overlap
between the CSI-RSs of the CSI-RS patterns in each of group A and
group B. That is, the CSI-RS patterns 0, 2, and 4 are designed such
that the CSI-RSs are arranged in different positions.
[0112] In a case where the CSI-RS patterns are grouped into g
groups and assigned according to the cell ID, the CSI-RS pattern
assignment can be determined as follows. The cell ID of the cell
assigned the CSI-RS pattern of group g is NCell_ID, and group g has
Ng CSI-RS patterns, the CSI-RS pattern of the corresponding cell is
determined by Equation (2).
CSI_RS Pattern ID of group g=NCell_ID mod Ng Equation (2)
[0113] In Equation (2), `CSI_RS Pattern ID of group g` is obtained
by grouping N.sub.g CSI-RS patterns into the group g and then
assigning indices from 0 to N.sub.g-1 to the CSI-RS patterns. For
example, if the group g includes the CSI-RS patterns 0, 2, and 4 of
FIG. 9, the CSI-RS pattern IDs of CSI-RS patterns included in the
group g become 0, 1, and 2.
[0114] FIG. 11 is a diagram illustrating CSI-RS patterns designed
to be assigned to a plurality of cells in a mobile communication
system according to an exemplary embodiment of the present
invention.
[0115] The CSI-RS patterns depicted in FIG. 11 are used to transmit
the CSI-RSs of a plurality antenna ports in multiple OFDM symbols
and are designed such that the CSI-RSs are transmitted on different
resources in the time and the frequency domains. In a case where
there are two CSI-RS patterns using the same OFDM symbol, the
CSI-RSs of the different antenna ports are alternately allocated
the resources different in the frequency domain.
[0116] Referring to FIG. 11, the six CSI-RS patterns defined in
FIG. 11 are assigned to different cells in a distributed manner to
avoid the overlap of the locations for the CSI-RS transmission by
as much as possible, as described with reference to FIG. 9. In FIG.
11, CSI-RS pattern 0 is composed of the REs different from those of
the CSI-RS patterns 2, 3, 4, and 5. For this reason, even though
the transmission powers of the CSI-RSs transmitted by other cells
with the CSI-RS patterns 2, 3, 4, and 5 are increased, the UE can
measure the channel with the CSI-RS transmitted by the cell using
the CSI-RS pattern 0 without additional interference. The reason
why the CSI-RS transmitted in the CSI-RS pattern 0 is not
interfered by the CSI-RSs transmitted in the CSI-RS patterns 2, 3,
4, and 5 is because the different time and frequency resources are
used, and this is achieved regardless of the number of CSI-RS
antenna ports. In an exemplary case where CSI-RSs are transmitted
for two antenna ports, it is possible to prevent the interference
from occurring by transmitting the CSI-RSs at the positions for the
antenna ports 0 and 1.
[0117] In FIG. 11, the CSI-RS pattern 0 and the CSI-RS pattern 1
may cause interference with each other. In a case where the number
of antennas of which CSI-RSs are transmitted in the CSI-RS pattern
0 is equal to or less than 4 and the number of antennas of which
CSI-RSs are transmitted in the CSI-RS pattern 1 is equal to or less
than 4, the CSI-RSs transmitted in the CSI-RS patterns 0 and 1 do
not interfere with each other. Interference occurs either when the
number of antennas of which CSI-RSs are transmitted in the CSI-RS
pattern 0 is greater than 4 or the number of antennas of which
CSI-RSs are transmitted in the CSI-RS pattern 1 is greater than 4.
The reason why the interference occurs in some limited cases is
because the CSI-RS patterns are designed such that the CSI-RSs
alternate in the same OFDM symbol. For example, when using the
CSI-RS pattern 0, the CSI-RSs of the antenna ports 0, 4, 2, and 4
are transmitted in the 9.sup.th OFDM symbol, and the CSI-RSs of the
antenna ports 5, 1, 7, and 3 are transmitted in the 10.sup.th OFDM
symbol. Meanwhile, when using the CSI-RS pattern 1, the CSI-RSs of
the antenna ports 5, 1, 7, and 3 are transmitted in the 9.sup.th
OFDM symbol, and the CSI-RSs of the antenna ports 0, 2, 4, and 6
are transmitted in the 10.sup.th OFDM symbol. In this manner, the
CSI-RSs of different antenna ports are transmitted in two different
OFDM symbols with the CSI-RS pattern 0 and CSI-RS pattern 1 such
that the transmission can be performed without interference when
the number of antenna ports is equal to or less than 4.
[0118] FIGS. 9 and 11 show the CSI-RS patterns available in one PRB
that are defined for the transmission of CSI-RSs, and Equations (1)
and (2) define the method for assigning the cell IDs to the cells
according to exemplary embodiments of the present invention. The
CSI-RSs are designed to be adoptable in various operation scenarios
in the LTE-A system. One of the significant operation scenarios of
the LTE-A system is the heterogeneous network having cells with
different sizes of service areas. In the heterogeneous network
environment, cells that are relatively small (i.e., a few meters in
diagonal) and large (i.e., a few kilometers in diagonal) in size
coexist in the same geographical area. In this case, it is required
to apply the CSI-RSs differently depending on the type of cell.
[0119] In order to provide different CSI-RSs depending on the type
of cell, an exemplary embodiment of the present invention proposes
a method for dividing one PRB into a plurality of regions and
defining a plurality of CSI-RS patterns in each region.
[0120] FIG. 12 is a diagram illustrating a PRB in which three
CSI-RS regions are defined according to an exemplary embodiment of
the present invention.
[0121] Referring to FIG. 12, the CSI-RS region A is used for
transmitting the CSI-RSs in the cell small in size, the CSI-RS
region B is used for transmitting the CSI-RSs in the cell
intermediate in size, and the CSI-RS region C is used for
transmitting the CSI-RSs in the cell large in size.
[0122] FIG. 13 is a diagram illustrating a principle for
transmitting CSI-RSs in a cellular environment including various
types of cells according to an exemplary embodiment of the present
invention. Referring to FIG. 13, it is shown that different types
of CSI-RSs are assigned to cells depending on the cell size. The
reason why the resource is divided into a plurality of CSI-RS
regions which are assigned depending on the characteristic of the
cell is to manage the CSI-RS resources efficiently by distributing
the same CSI-RS resource to the cells having similar
characteristics.
[0123] The CSI-RS resource allocation based on the type of the cell
as shown in FIG. 13 can be performed as follows:
[0124] 1. The eNB divides the given CSI-RS transmission resource
into a plurality of CSI-RS regions.
[0125] 2. The eNB determines the type of cells to which each CSI-RS
region is assigned.
[0126] 3. The eNB determines the type of the corresponding cell
(e.g., Femto, Micro, and Macro).
[0127] 4. The eNB determines the CSI-RS pattern to be used in the
CSI-RS region based on the type of cell and allocates the CSI-RS
pattern to the cell.
[0128] Here, the CSI-RS resource allocation can be performed in
association with the CSI-RS patterns depicted in FIGS. 9 and 11.
For example, when the last two OFDM symbols are defined as CSI-RS
region A and the rest as CSI-RS region B in FIG. 11, the CSI-RS
pattern E and CSI-RS pattern F are assigned only to the cells
classified so as to use the CSI-RS region A.
[0129] FIGS. 14a to 14c are diagrams illustrating CSI-RS patterns
designed to be assigned to a plurality of cells in a mobile
communication system according to an exemplary embodiment of the
present invention.
[0130] More specifically, FIG. 14a shows three CSI-RS patterns
designed having CSI-RS transmission positions without overlapping
for supporting eight CSI-RS antenna ports. FIG. 14b shows six
CSI-RS patterns designed having CSI-RS transmission positions
without overlapping for supporting four CSI-RS antenna ports. FIG.
14c shows twelve CSI-RS patterns designed having CSI-RS
transmission positions without overlapping for supporting two
CSI-RS antenna ports. The CSI-RS patterns depicted in FIGS. 14a to
14c are characterized in that the number of available CSI-RS
patterns increases as the number of CSI-RS antenna ports
decreases.
[0131] Referring to FIGS. 14a to 14c, the number of available
CSI-RS patterns varies depending on the number of antenna ports. In
this case, the method for determining the CSI-RS for each cell
varies. In an exemplary case where the number of antenna ports is
eight or four as shown in FIG. 9 or FIG. 11 and six CSI-RS patterns
are defined, it is possible to determine the CSI-RS pattern ID
using Equation (1) regardless of the number of antenna ports. When
the number of antennas is eight, four, and two as shown in FIGS.
14a to 14c, three, six, and twelve CSI-RS patterns are defined,
respectively. In this case, the CSI-RS pattern is determined as
follows:
[0132] Step 1. The eNB notifies the UE of the number of CSI-RS
antennas of the connected cell.
[0133] Step 2. The UE determines the CSI-RS pattern that can
support the number of CSI-RS antennas using Equation (3).
CSI_RS Pattern ID=NCell_ID mod NA Equation (3)
[0134] In Equation (3), the value of NA varies depending on the
number of CSI-RS antenna ports. The value of NA is 12 when the
number of CSI-RS antennas is 2, 6 when the number of CSI-RS
antennas is 4, and 3 when the number of CSI-RS antennas is 8.
[0135] In order to determine the CSI-RS pattern using Equation (3),
the eNB and UE have to share information and, in this case, the UE
can determine the CSI-RS pattern to use based on the number of
CSI-RS ports that is notified by the eNB. This method can be
applied to the NA having a value and typically when the NA is
identical according to the number of CSI-RS antenna ports.
[0136] FIG. 15 is a diagram illustrating a principle for
transmitting CSI-RSs in a plurality of cells without interference
according to an exemplary embodiment of the present invention.
[0137] Referring to FIG. 15, the CSI-RSs are transmitted in
different subframes with different time offset values in respective
cells. The time offset of the CSI-RS transmitted by each cell can
be shared between the eNB and UE with one of two methods. First,
the eNB determines the time offset value per cell in advance and
transmits the time offset value in the form of control information
through higher layer signaling. Second, the eNB and UE generates
the time offset value using a previously negotiated method.
[0138] In FIG. 15, the eNB of cell 1 transmits a CSI-RS in a
subframe 1210, the eNB of cell 2 transmits a CSI-RS in a subframe
1220, and the eNB of cell 3 transmits a CSI-RS in a subframe
1230.
[0139] It is advantageous for the multiple cells to transmit the
CSI-RSs with the time offset value of different subframes since the
CSI-RSs transmitted by the different cells do not interfere with
each other. In a case of LTE and LTE-A systems operating in a Time
Division Duplex (TDD) mode, however, it can be limited to use the
subframe for downlink transmission in the radio frame composed of
10 subframes. In this case, the CSI-RS must be transmitted in some
specific subframes and as a consequence it becomes difficult to
sufficiently distribute the CSI-RSs transmitted by a plurality of
cells in a unit of a subframe. In a case where the CSI-RSs are not
distributed sufficiently in a unit of a subframe, it is possible to
reduce the interference between the CSI-RSs transmitted in the
plural cells by assigning the CSI-RS patterns, as shown in FIG. 11,
to the cells. This means that the CSI-RSs of the different cells
are transmitted in different CSI-RS patterns in the same
subframe.
[0140] FIG. 16 is a diagram illustrating a principle for
transmitting CSI-RSs of multiple cells with different CSI-RS
patterns in a subframe according to an exemplary embodiment of the
present invention.
[0141] Referring to FIG. 16, cell 1 transmits the CSI-RSs in the
subframes denoted by reference number 1320, but not in the
subframes denoted by reference number 1330. Cell 2 transmits the
CSI-RSs in the subframes denoted by reference number 1350 that are
identical with those in which cell 1 transmits its CSI-RSs, but not
in the subframes denoted by reference number 1360. In FIG. 16, cell
2 transmits the CSI-RSs in the same subframe as cell 1 does but the
CSI-RS pattern used by cell 2 (i.e., the CSI-RS pattern denoted by
reference number 1340) differs from the CSI-RS pattern used by cell
1 (i.e., the CSI-RS pattern denoted by reference number 1310),
thereby avoiding interference between CSI-RSs.
[0142] In the LTE-A system, the CSI-RS is used to measure the state
of the downlink channel of the cell to which the UE belongs. The UE
measures the downlink channel of the corresponding cell using the
CSI-RS transmitted by one cell, but it is also possible for the UE
to measure the downlink channels using the CSI-RSs transmitted by
two or more cells. Measurement of the downlink channels of the
CSI-RSs transmitted by multiple cells can be performed when the UE
receives signals in a Coordinated Multi Point (CoMP) transmission
scheme. In a case of CoMP transmission, multiple eNBs cooperate for
transmission to a single UE. At this time, the eNBs perform
precoding in consideration of the precoding of other eNBs and
transmit signals to the UE simultaneously.
[0143] In order for the multiple eNBs to support the CoMP
transmission to a single UE, the UE must have the capacity to
measure the channel states of the eNBs to be involved to the CoMP
transmission. That is, the UE can measure the CSI-RSs of the
multiple cells and transmit the measurement results to the
corresponding eNBs. An exemplary embodiment of the present
invention proposes a novel CSI-RS transmission method for CoMP
transmission. The CSI-RS transmission method for CoMP according to
an exemplary embodiment of the present invention is characterized
by transmitting the CoMP CSI-RS and non-CoMP CSI-RS in different
time points. That is, the CSI-RSs of individual cells are
distinguished between CoMP CSI-RS and non-CoMP CSI-RS according to
the subframe carrying the CSI-RSs.
[0144] FIG. 17 is a diagram illustrating a principle for
transmitting CoMP CSI-RS and non-CoMP CSI-RS in a cell of a mobile
communication system according to an exemplary embodiment of the
present invention.
[0145] In FIG. 17, the non-CoMP CSI-RS or the CoMP CSI-RS is
transmitted at a first time interval, the CoMP CSI-RS are
transmitted once within a second time interval and the non-CoMP
CSI-RSs are transmitted at the first time interval. Here, the first
time interval can be the time duration corresponding to 5
subframes, and the second time interval can be the time duration
corresponding to 15 subframes.
[0146] Referring to FIG. 17, the non-CoMP CSI-RS is transmitted in
the subframes patterns as denoted by reference number 1420, and the
CoMP CSI-RS is transmitted in the subframes colored as denoted by
reference number 1410. In the subframes that are not marked, such
as the subframes denoted by reference number 1430, no CSI-RS is
transmitted. In the exemplary structure of FIG. 17, the CSI-RS is
transmitted at every 5 msec among the CSI-RSs transmitted at every
5 msec, the CoMP CSI-RS is transmitted at every 15 msec, and the
rest CSI-RSs are non-CSI-RSs. In order to transmit the CoMP CSI-RSs
and the non-CoMP CSI-RSs as shown in FIG. 17, the subframes to
carry the CSI-RSs should be determined and assigned the subframes
for carrying the CoMP CSI-RS and the non-CoMP CSI-RS. The method
for transmitting the CoMP CSI-RSs and non-CoMP CSI-RSs as described
with reference to FIG. 17 is advantageous to transmit the CoMP
CSI-RSs and non-CoMP CSI-RSs with the radio resources constantly
allocated for the CSI-RS transmission.
[0147] In order to transmit the CoMP CSI-RS and non-CoMP CSI-RS as
shown in FIG. 17, the eNB notifies the UE of the CSI-RS
transmission interval and the rule for distinguishing between the
subframe carrying the CoMP CSI-RS and the subframe carrying the
non-CoMP CSI-RS.
[0148] FIG. 18 is a diagram illustrating a principle for
transmitting CoMP CSI-RS and non-CoMP CSI-RS in a cell of a mobile
communication system according to an exemplary embodiment of the
present invention.
[0149] Referring to FIG. 18, the non-CoMP CSI-RS is transmitted at
a first time interval, and the CoMP CSI-RS at a second time
interval, the second time interval being greater than the first
time interval. Here, the first time interval is equal to 5 subframe
durations, and the second time interval is equal to 15 subframe
durations.
[0150] As shown in FIG. 18, the non-CoMP CSI-RS is transmitted in
the subframes patterned as denoted by reference number 1520, and
the CoMP CSI-RS is transmitted in the subframes colored as denoted
by reference number 1510. In the subframes that are not marked,
such as the subframes denoted by reference number 1530, no CSI-RS
is transmitted. In FIG. 18, it is shown that the time durations for
transmitting the CoMP CSI-RS and the non-CoMP CSI-RS are separately
provided. Unlike the method of FIG. 17, which determines the time
durations for transmitting the CSI-RS first and then allocates the
determined time durations for transmitting the CoMP CSI-RS and
non-CoMP CSI-RS, the method of FIG. 18 determines the time
durations for transmitting the CoMP CSI-RS and non-CoMP CSI-RS
separately. As a consequence, the CoMP CSI-RS is transmitted at
every 15 msec, and the non-CoMP CSI-RS at every 5 msec in FIG.
18.
[0151] FIGS. 17 and 18 show the methods for transmitting the CoMP
CSI-RS and non-CoMP CSI-RS at predetermined transmission time
durations in a cell. It is determined which type of CSI-RS is
transmitted in which subframe. In addition, how to generate the
CoMP CSI-RS is important.
[0152] The CoMP CSI-RS proposed in an exemplary embodiment of the
present invention is transmitted in a different manner as compared
to the non-CoMP CSI-RS. That is, the eNB generates different
signals depending on whether the CSI-RS to be transmitted in the
cell is a CoMP CSI-RS. The CoMP CSI-RS proposed in an exemplary
embodiment of the present invention is transmitted using a smaller
number of virtual antenna ports as compared to the non-CoMP CSI-RS.
Antenna virtualization refers to transmitting, when N physical
antennas exist, the same signal such that the signal is shown as if
being transmitted by M antennas. For example, if a signal is
transmitted by two antennas at the transmission powers P1 and P2,
the signal is received by the receiver as if being transmitted by
one transmit antenna at the transmission power of P1+P2.
[0153] FIG. 19 is a diagram illustrating a principle for
transmitting CoMP CSI-RS and non-CoMP CSI-RS in a mobile
communication system according to an exemplary embodiment of the
present invention. FIG. 19 is depicted under the assumption that
two cells are transmitting CSI-RSs.
[0154] Referring to FIG. 19, the subframe patterned as denoted by
reference number 1620 carries the non-CoMP CSI-RS transmitted by
cell 1, the subframe colored as denoted by reference number 1640
carries the CoMP CSI-RS transmitted by cell 1, and the subframe not
marked as denoted by reference number 1630 carries no CSI-RS. The
subframe patterned as denoted by reference number 1660 carries the
non-CoMP CSI-RS transmitted by cell 2, the subframe colored as
denoted by reference number 1680 carries the CoMP CSI-RS
transmitted by cell 2, and the subframe not marked as denoted by
reference number 1695 carries no CSI-RS.
[0155] In FIG. 19, the subframe for transmitting the non-CoMP
CSI-RS carries the CSI-RSs arranged as denoted by reference numbers
1610 and 1670 that are transmitted by cell 1 and cell 2. The CSI-RS
patterns for transmitting the non-CoMP CSI-RS in the subframes
denoted by reference numbers 1610 and 1670 are ones of the six
CSI-RS patterns depicted in FIG. 11. In a case where the non-CoMP
CSI-RS is transmitted, each cell transmits the CSI-RS which is
capable of channel measurement to all antenna ports. Otherwise, in
a case where the CoMP CSI-RS is transmitted as denoted by reference
numbers 1650 and 1690 of FIG. 19, each cell transmits the CSI-RS of
the reduced number of antenna ports. The method for reducing the
number of antenna ports of the CSI-RSs transmitted, as denoted by
reference number 1650 and 1690 of FIG. 19, can be performed using
the aforementioned antenna virtualization. In a case of
transmitting the CoMP CSI-RS in this manner, the number of antenna
ports of CSI-RS is reduced and thus the number of antenna ports of
CSI-RSs transmitted by each cell decreases, but the transmission
power of each antenna increases. That is, in a case denoted by
reference numbers 1650 and 1690 of FIG. 19, the CoMP CSI-RS is
transmitted at a transmission power that is twice that of the
non-CSI-RS.
[0156] The reason why the CoMP CSI-RS is transmitted as shown in
FIG. 19 is to improve the reception performance of the UEs having
bad received signal strength at a cell edge. That is, if the cells
transmit the CoMP CSI-RS, it is possible to reduce the number of
antenna ports of each cell which the UE would measure and increase
the transmission power at each antenna port, resulting in an
improvement of the reception performance of UE. In a case where the
UEs are located at the cell edge, it is advantageous to increase
the received signal strength of each layer rather than to increase
the spatial multiplexing gain with an increase in the number of
layers. In a case of transmitting the CoMP CSI-RS separately, as
shown in FIG. 19, without reducing the antenna ports, the UE
receives a number of CSI-RSs corresponding to the number of cells
involved in the CoMP. That is, assuming that the UE receives
signals from cell 1 and cell 2 in CoMP, the UE must measure a total
of 16 antenna ports and feed back the channel measurement
information to the eNB. In the situation where the large number of
layers is not advantageous for receiving signal, it is inefficient
to perform channel measurement on the increased number of antenna
ports to the non-CoMP and report a relatively large amount of
channel measurement information.
[0157] In a case where the CSI-RSs are transmitted by multiple
cells, it is possible to avoid the collision between the CSI-RSs of
different cells with multiple CSI-RS patterns as depicted in FIGS.
9 and 11. Another method for improving the CSI-RS reception
performance is to use muting or blanking. Muting refers to not
transmitting a signal at a specific RE position. That is, at the
REs on which cell 1 transmits the CSI-RS, cell 2 does not transmit
any signal in order for the UE to measure the CSI-RS transmitted by
cell 1 more accurately.
[0158] FIG. 20 is a diagram illustrating a principle of
transmitting CSI-RS with a muting scheme in a mobile communication
system according to an exemplary embodiment of the present
invention.
[0159] Referring to FIG. 20, multiple cells transmit CSI-RSs in
different CSI-RS patterns, and each cell determines the CSI-RS
patterns used by neighbor cells and mutes the resources on which
the neighbor cells transmit their CSI-RSs.
[0160] FIG. 20 is depicted under the assumption that cell 1
transmits CSI-RSs in the CSI-RS pattern A of FIG. 11, and cell 2,
which is adjacent to cell 1, transmits CSI-RSs in the CSI-RS
pattern C of FIG. 11. In this case, the CSI-RS pattern of cell 1
has the instances as denoted by reference numbers 1710, 1730, and
1760. The CSI-RSs are transmitted at the REs assigned to the
corresponding antenna ports according to a number of CSI-RS antenna
ports of cell 1. In a case where cell 2 transmits the CSI-RSs in
the CSI-RS pattern C of FIG. 11, the signals transmitted by cell 1
are muted according to the number of CSI-RS antenna ports of cell
2. If the number of CSI-RS antenna ports of cell 2 is 2, cell 1
performs muting on the resources denoted by reference number 1720
to help the channel measurement based on the CSI-RS of cell 2. If
the number of the CSI-RS antenna ports of cell 2 is 4, cell 1
performs muting on the resources denoted by reference numbers 1740
and 1750. Also, if the number of the CSI-RS antenna ports of cell 2
is 8, the cell performs muting on the resources as denoted by
reference numbers 1770 and 1780.
[0161] FIG. 21 is a diagram illustrating a principle of
transmitting CSI-RS with a muting scheme in a mobile communication
according to an exemplary embodiment of the present invention. FIG.
21 shows the resources on which cell 2 performs muting when cell 1
transmits CSI-RS.
[0162] Referring to FIG. 21, cell 1 transmits CSI-RSs, and cell 2
performs muting at the resources carrying the CSI-RS of cell 1 to
help channel measurement base on the CSI-RS of cell 2. Also, cell 2
transmits CSI-RS and, on the resources carrying the CSI-RS of cell
1, performs muting to help channel measurement based on the CSI-RS
of cell 1. Herein, reference numbers 1810, 1820, 1830, 1840, 1850,
1860 and 1870 are similar to reference numbers 1710, 1720, 1730,
1740, 1750, 1760 and 1770 of FIG. 20 and therefore their
description will be omitted herein for conciseness in
explanation.
[0163] As shown in FIGS. 20 and 21, when multiple cells transmit
CSI-RSs, each cell performs muting on the REs carrying the CSI-RS
of the other cells to help the UE estimate channels based on the
CSI-RSs of the other cells. In order to adopt the muting scheme,
the cells must share the information on the CSI-RS patterns. That
is, in order to perform muting transmission of CSI-RS, the eNB
transmitting the CSI-RS to a specific cell as shown in FIG. 20
should know the information on the CSI-RS patterns used by the eNB
transmitting the CSI-RS as shown in FIG. 21 and the number of
antenna ports of which CSI-RS are transmitted. In a case of
adopting the muting scheme as shown in FIG. 21 to the CSI-RS
transmission as shown in FIG. 17, the involved cells must share
their CSI-RS patterns and numbers of CSI-RS antenna ports.
[0164] FIGS. 20 and 21 show the muting scheme performed in the
subframe in which the CSI-RS are transmitted. However, the muting
scheme can be used in the subframe in which no CSI-RS exists, in
the same manner. Also, the muting scheme is not applied in the
subframe in which no CSI-RS of other cells exist, but partially in
such a subframe.
[0165] FIG. 22 is a diagram illustrating a principle for applying a
muting scheme to a case where multiple cells transmit CSI-RSs with
different subframe offsets according to an exemplary embodiment of
the present invention.
[0166] Referring to FIG. 22, two cells are transmitting CSI-RSs,
and the CSI-RSs of the respective cells are transmitted with
different subframe offsets. The muting scheme is determined
depending on the transmission pattern of CSI-RSs in the two cells.
That is, cell 1 performs muting at the resources denoted by
reference number 1930 in the subframe 1915 in FIG. 22. The reason
why the signal transmitted by cell 1 is muted in the subframe 1915
is to help the UE measure the CSI-RS transmitted by the cell as
denoted by reference number 1965.
[0167] In FIG. 22, it is observed that the muting is applied not to
the entire resource of the subframe in which the CSI-RSs are
transmitted but to a part of the subframe. Applying the muting
partially is to reduce the performance degradation of data signal
that can occur due to the muting. In FIG. 22, it is also observed
that the CSI-RS occurs at every 5 subframes and the muting occurs
at every 20 subframes. In order to transmit the CSI-RSs in the
manner of FIG. 22, one cell should know the subframe offset and
transmission interval of CSI-RS in the other cell.
[0168] In FIGS. 20, 21, and 22, the muting scheme is applied under
the assumption that the CSI-RSs are transmitted in all the PRBs.
However, the muting scheme can be applied to the case where the
CSI-RSs are transmitted in only some of the PRBs, in the same
manner.
[0169] FIG. 23 is a diagram illustrating a principle for applying a
muting scheme applied to a case where two cells transmit CSI-RS in
some PRBs according to an exemplary embodiment of the present
invention.
[0170] Referring to FIG. 23, cell 2 applies muting to the resources
as denoted by reference number 2020 while cell 1 transmits CSI-RS
on the same resources as denoted by reference number 2010, and cell
1 applies muting to the resources as denoted by reference number
2030 while cell 2 transmits CSI-RS on the same resources as denoted
by reference number 2040.
[0171] FIG. 24 is a flowchart illustrating a method for a UE to
receive CSI-RS in a mobile communication system operating with a
plurality of CSI-RS patterns depicted in FIG. 9 or FIG. 11
according to an exemplary embodiment of the present invention.
[0172] Referring to FIG. 24, the UE determines the cell ID of the
eNB which performs transmission in the corresponding cell in step
2105, and then determines CRS positions and scrambling and receives
the system information related to CSI-RS in step 2110. Here, the
cell ID is used for determining the information on the CRS position
and scrambling, and the UE obtaining the CRS information can
receive the system information of the corresponding cell. The
system information received at step 2110 includes control
information related to CSI-RS. The UE can receive the control
information on the CSI-RS from the eNB directly at step 2110. In a
case where the CSI-RS pattern for use in the transmission of CSI-RS
of a specific cell is a function of the cell ID as described with
reference to FIGS. 9 and 11, the UE determines the cell ID of the
corresponding eNB to obtain the system information related to the
CSI-RS transmission such as CSI-RS position and scrambling.
[0173] Afterward, the UE receives and processes subframes. If a
subframe is received, the UE determines whether the subframe is
carrying the CSI-RS based on the information obtained at step 2110
in step 2125. If it is determined that the subframe is carrying the
CSI-RS, the UE measures a downlink channel state using the CSI-RS
in step 2130. Next, the UE determines whether the subframe carries
a Physical Downlink Shared Channel (PDSCH) destined to itself in
step 2135. If it is determined that the subframe carries the PDSCH,
the UE determines whether CSI-RS exists on the frequency resource,
i.e., PRB, where the PDCCH is transmitted in step 2140. If a CSI-RS
exists in the PRB allocated for transmitting the PDSCH, the UE
performs a PDSCH reception operation in consideration of the CSI-RS
not existing in the allocated PRB in step 2145 and waits for the
next subframe in step 2115.
[0174] If it is determined that the subframe does not carry the
PDSCH at step 2135, the UE waits for the next subframe in step
2115. Also, if there is no CSI-RS in the PRB allocated for
transmitting the PDSCH (i.e., if no subframe carrying a CSI-RS is
received) at step 2140, the UE performs the PDSCH reception
operation in consideration of the CSI-RS not existing in the
allocated PRB in step 2155 and waits for the next subframe in step
2120. If it is determined that the subframe is not carrying the
CSI-RS at step 2125, the UE determines whether the subframe is
carrying a PDSCH in step 2150 and, if a PDSCH is contained in the
subframe, performs the PDSCH reception operation in step 2155 and
waits for the next subframe in step 2120. If no PDSCH is contained
in the subframe at step 2150, the UE returns the process to step
2120 to wait for the next subframe.
[0175] FIG. 25 is a flowchart illustrating a method for a UE to
receive CSI-RS in a mobile communication system operating with a
plurality of CSI-RS patterns depicted in FIG. 9 or FIG. 11 with a
muting scheme according to an exemplary embodiment of the present
invention.
[0176] Referring to FIG. 25, the UE determines the cell ID of the
eNB which performs transmission in the corresponding cell in step
2205, and then determines CRS positions and scrambling and receives
the system information related to CSI-RS in step 2210. Here, the
cell ID is used for determining the information on the CRS position
and scrambling, and the UE obtaining the CRS information can
receive the system information of the corresponding cell. The
system information received at step 2210 includes control
information related to CSI-RS. The UE can receive the control
information on the CSI-RS from the eNB directly at step 2210. The
control information which the UE has received from the eNB includes
the muting position of the eNB as well as the information on the
CSI-RS of the cell to which the UE belongs. Muting can be performed
in two manners. In the first manner, the eNB notifies the UE of the
muting positions directly, and in the second manner, the eNB
notifies the UE of the CSI-RS pattern to which muting is applied.
In a case where there are a plurality of CSI-RS patterns defined
between the eNB and UE, the eNB needs only to notify the UE of
which CSI-RS pattern is used for muting. Here, the muting control
information transmitted from the eNB to the UE includes the
corresponding CSI-RS pattern, number of antennas to be muted,
muting interval, subframe offset of muting, etc.
[0177] After determining the muting control information at step
2210, the UE receives a subframe and determines whether the
subframe is supposed to be muted in step 2225. If it is determined
that the subframe is supposed to be muted, the UE determines
whether the subframe carries a PDSCH destined for itself in step
2230. If the subframe carries the PDSCH, the UE determines whether
the PRB allocated to the UE has a muted part in step 2235. The UE
uses the muting control information to determine whether the muted
part exists. If there is no muted part in the PRB, the UE performs
a PDSCH reception operation in consideration of the muted part not
existing in the allocated PRB in step 2250. Otherwise, if there is
a muted part in the PRB at step 2235, the UE performs a PDSCH
reception operation in consideration of the muted part existing in
the allocated PRB in step 2240. That is, the UE performs receiving
of signals under the assumption that no signal is received in the
muted part. After processing the PDSCH at step 2240, the UE waits
for the next subframe in step 2215. Also, after processing the
PDSCH at step 2250, the UE waits for the next subframe in step
2220. If it is determined that the subframe is not supposed to be
muted at step 2225, the UE determines whether the subframe carries
a PDSCH destined for itself in step 2245. If the subframe carries
the PDSCH at step 2245, the process proceeds to step 2250,
otherwise the process proceeds to step 2220. If the subframe does
not carry the PDSCH at step 2230, the process proceeds to step
2215.
[0178] FIG. 26 is a diagram illustrating a method for an eNB to
transmit CSI-RS with a muting scheme in a mobile communication
system according to an exemplary embodiment of the present
invention.
[0179] Referring to FIG. 26, an eNB exchanges information related
to the transmission of CSI-RS with neighbor eNBs in step 2305. At
step 2305, the eNBs coordinate the CSI-RS transmission per cell as
well as exchange the information. Here, the coordination on the
CSI-RS transmission refers to the distribution of the CSI-RS
patterns defined as shown in FIGS. 9 and 11 to transmit the
CSI-RSs. After the information exchange, the eNB determines muting
positions in downlink transmission using the information related to
the CSI-RS transmission in step 2310. That is, the eNB determines
how to apply muting in detail by determining the muting interval,
duration, and position in frequency domain. Next, the eNB transmits
control information related to the CSI-RS and muting to the eNB in
step 2320. At this time, the control information related to the
CSI-RS and muting can be transmitted in the form of system
information. After transmitting the information about the CSI-RS
and muting, the eNB determines whether a current subframe is
supposed to carry the CSI-RS in step 2330.
[0180] If the current subframe is supposed to carry the CSI-RS, the
eNB transmits the CSI-RS in the current subframe in step 2335.
Next, the eNB determines whether the current subframe is supposed
to receive muting in step 2340 and, if the current subframe is
supposed to take muting, performs downlink transmission of the
subframe with muting in step 2345. That is, the eNBs share the
CSI-RS patterns and number of CSI-RS antenna ports about adjacent
cells. At the time to transmit its subframe, the eNB analyzes the
CSI-RS pattern to determine whether the subframe is supposed to
carry CSI-RS and, if the CSI-RS transmission time arrives,
transmits the CSI-RS in the subframe based on the CSI-RS pattern.
Also, if the CSI-RS transmission time of the adjacent cell arrives,
the eNB performs muting on a number of REs corresponding to the
number of CSI-RS antenna ports of the CSI-RS pattern of the
adjacent cell. Thereafter, the eNB waits for the next subframe in
step 2325, and when there is the next subframe, the process returns
to step 2330. Accordingly, this operation repeats in every
subframe.
[0181] FIG. 27 is a diagram illustrating a method for an eNB to
transmit non-CoMP CSI-RS and CoMP CSI-RS in a mobile communication
according to an exemplary embodiment of the present invention.
[0182] Referring to FIG. 27, the eNB determines the cells belonged
to a CoMP set in step 2405. The CoMP set is a set of a plurality of
cells involved in the CoMP transmission. After determining the
cells participating in the CoMP, the eNB shares the control
information related to the CoMP CSI-RS transmission with the cells
belonged to the CoMP set in step 2410. In a case where the CoMP
CSI-RS is transmitted as shown in FIG. 19, the shared information
can include the CSI-RS patterns in which the CoMP CSI-RS of the
cells are transmitted and antenna ports and scrambling to be used
for transmission. Afterward, the eNB notifies the UEs of the
information on the CoMP CSI-RS and non-CoMP CSI-RS in step
2415.
[0183] Next, the eNB determines whether the current subframe is
supposed to carry the CSI-RS in step 2425. If the current subframe
is supposed to carry the CSI-RS, the eNB determines whether the
current subframe is supposed to carry the CoMP CSI-RS in step 2430.
If the current subframe is supposed to carry the CoMP CSI-RS, the
eNB transmits the CoMP CSI-RS in the subframe in step 2240 and,
otherwise, the eNB transmits the non-CoMP CSI-RS in the subframe in
step 2435. If the current subframe is not supposed to carry the
CoMP CSI-RS at step 2425 and after transmitting the CSI-RS at step
2440 or 2435, the eNB proceeds to step 2420 to wait for the next
subframe. Thereafter, the process returns to step 2425.
[0184] As described above, if the current subframe is supposed to
carry CSI-RS, the eNB determines whether the CSI-RS to be
transmitted is CoMP CSI-RS. At this time, if the current subframe
is supposed to carry CoMP CSI-RS, the eNB transmits the CoMP CSI-RS
and, otherwise, non-CoMP CSI-RS. Here, the CoMP CSI-RS is the
CSI-RS transmitted by multiple cells for the UE to measure the
downlink channels of the multiple cells, and the CoMP set is a set
of the cells participated in the CoMP CSI-RS transmission. Here,
the eNB transmits the CSI-RSs in the subframes positioned at a
first time interval and, among the CSI-RSs, the CoMP CSI-RSs are
transmitted in the subframes at a second time interval and the
non-CoMP CSI-RSs are transmitted in the rest subframes at the first
time interval, where the second time interval can be multiple of
the first time interval. When transmitting the CSI-RS, it can be
designed such that the non-CoMP CSI-RSs are transmitted in the
subframes at the first time interval and the CoMP CSI-RSs are
transmitted in the subframes at the second time interval, where the
second time interval is multiple of the second time interval in
length and the subframes carrying the non-CoMP CSI-RSs and CoMP
CSI-RSs are not overlapped. Here, the CoMP CSI-RSs use a small
number of the virtual antenna ports as compared to the non-CoMP
CSI-RSs, and the first time interval can be equal to 5
subframes.
[0185] FIG. 28 is a flowchart illustrating a method for a UE to
receive a non-CoMP CSI-RS and a CoMP CSI-RS in a mobile
communication according to an exemplary embodiment of the present
invention.
[0186] Referring to FIG. 28, the UE determines the cell ID of the
eNB transmitting signals in the corresponding cell in step 2505.
The cell ID is used to acquire the information on the CRS positions
and scrambling, and the UE receives the system information of the
corresponding cell based on the CRS information in step 2510. The
UE determines the CRS position and scramble and then receives the
information related to the CoMP CSI-RS and non-CoMP CSI-RS at step
2510. The system information of the corresponding cell includes the
control information used for receiving the CoMP CSI-RS and
non-CSI-RS. The UE determines, based on the CoMP CSI-RS and
non-CoMP CSI-RS information acquired at step 2510, whether the
current subframe carries a CSI-RS in step 2520. If the subframe
carries a CSI-RS at step 2520, the UE determines whether the CSI-RS
carried in the subframe is CoMP CSI-RS in step 2525. If the CSI-RS
carried in the subframe is CoMP CSI-RS, the UE measures the CoMP
channels based on the scrambling sequence and antenna ports of each
cell in step 2530. Next, the UE generates the CoMP channel state
information based on the measurement and reports the channel
information to the eNB in step 2535. If the CSI-RS carried in the
subframe is non-CoMP CSI-RS at step 2525, the UEs measures the
channel state of the corresponding cell using the CSI-RS based on
the CSI-RS pattern, antenna port, and scrambling information
transmitted by the corresponding cell in step 2540. Next, the UE
generates the channel state information based on the measurement
and reports the channel state information to the eNB in 2545. If
the current subframe is not supposed to carry a CSI-RS at step 2520
and after steps 2535 or 2545, the UE proceeds to step 2515 to wait
for the next subframe. Thereafter, the process returns to step
2520. Accordingly, the UE repeats this operation for every
subframe.
[0187] As described above, the method for processing CSI-RS
according to an exemplary embodiment of the present invention is
capable of transmitting the CSI-RSs of antenna ports alternately in
different PRBs so as to manage transmission power of all antenna
ports of an eNB efficiently. The method for processing CSI-RS
according to an exemplary embodiment of the present invention
assigns different CSI-RS patterns to the cells in order to avoid
the transmission positions of the CSI-RSs of different cells,
thereby suppressing signal interferences, transmitting non-CoMP
CSI-RS and CoMP CSI-RS efficiently, and allowing the UE to measure
a channel efficiently with the assistance of muting the resource on
which a neighbor eNB transmits CSI-RS.
[0188] While the invention has been shown and described with
reference to certain exemplary embodiments thereof, it will be
understood by those skilled in the art that various changes in form
and details may be made therein without departing from the spirit
and scope of the invention as defined in the appended claims and
their equivalents.
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